On Mon, Oct 07, 2019 at 06:34:43PM +0200, Daniel Wagner wrote: > On Mon, Oct 07, 2019 at 06:23:30PM +0200, Uladzislau Rezki wrote: > > Hello, Daniel, Sebastian. > > > > > > On Fri, Oct 04, 2019 at 06:30:42PM +0200, Sebastian Andrzej Siewior wrote: > > > > > On 2019-10-04 18:20:41 [+0200], Uladzislau Rezki wrote: > > > > > > If we have migrate_disable/enable, then, i think preempt_enable/disable > > > > > > should be replaced by it and not the way how it has been proposed > > > > > > in the patch. > > > > > > > > > > I don't think this patch is appropriate for upstream. > > > > > > > > Yes, I agree. The discussion made this clear, this is only for -rt > > > > trees. Initially I though this should be in mainline too. > > > > > > Sorry, this was _before_ Uladzislau pointed out that you *just* moved > > > the lock that was there from the beginning. I missed that while looking > > > over the patch. Based on that I don't think that this patch is not > > > appropriate for upstream. > > > > > Yes that is a bit messy :) Then i do not see what that patch fixes in > > mainline? Instead it will just add an extra blocking, i did not want that > > therefore used preempt_enable/disable. But, when i saw this patch i got it > > as a preparation of PREEMPT_RT merging work. > > Maybe I should add some background info here as well. Currently, I am > creating an -rt tree on v5.3 for which I need this patch (or a > migrate_disable() version of it). So this is slightly independent of > the work Sebiastian is doing. Though the mainline effort of PREEMPT_RT > will hit this problem as well. > > I understood Sebiastian wrong above. I thought he suggest to use the > migrate_disable() approach even for mainline. > > I supppose, one thing which would help in this discussion, is what do > you gain by using preempt_disable() instead of moving the lock up? > Do you have performance numbers which could justify the code? > Actually there is a high lock contention on vmap_area_lock, because it is still global. You can have a look at last slide: https://linuxplumbersconf.org/event/4/contributions/547/attachments/287/479/Reworking_of_KVA_allocator_in_Linux_kernel.pdf so this change will make it a bit higher. From the other hand i agree that for rt it should be fixed, probably it could be done like: ifdef PREEMPT_RT migrate_disable() #else preempt_disable() ... but i am not sure it is good either. -- Vlad Rezki
